In producing oil from wells, it has become important to provide a safety valve for use in high temperature environments. For example, in order to increase recovery, it is conventional to inject steam into a well to heat and thin the oil for increasing recovery. However, high temperatures are involved, such as 800 degrees F. Generally, an all metal safety valve such as disclosed in U.S. Pat. No. 4,527,630 would be sufficient with the exception of the lubricants necessary to operate the piston actuating means. Most lubricating fluids will not exist in a liquid state at extremely high temperatures to provide the desired lubricity needed for the metal piston and cylinder assembly.
The present invention is directed to a subsurface safety valve having a fluid actuator using a ceramic piston movable in a cylinder having a ceramic interior. A ceramic piston and cylinder need not provide a positive seal, but only resistance to fluid flow sufficient to actuate the safety valve. However, the ceramic piston and cylinder will withstand the extremely high temperatures without lubricants. In order to provide positive sealing, two spaced metal valve seats coact with two metal valve elements connected between the piston and cylinder for sealing the ceramic piston and cylinder assembly at opposite ends of travel.